The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Modern nacelles are designed to house a dual flow turbojet engine capable of using rotating fan blades to generate a hot air flow (primary flow) coming from the combustion chamber of the turbojet engine, as well as a set of related actuating devices connected to its operation and performing various functions when the turbojet engine is operating or stopped.
The nacelle generally has an outer structure, called Outer Fixed Structure (OFS), which defines, with a concentric inner structure, called Inner Fixed Structure (IFS), a stream aiming to channel a cold air flow, called secondary flow, that circulates outside the turbojet engine.
The primary and secondary flows are discharged from the turbojet engine through the rear of the nacelle.
Furthermore, a nacelle generally has a tubular structure comprising an air intake upstream of the turbojet engine, a middle section designed to surround the fan of the turbojet engine, a downstream section integrating thrust reversal means and designed to surround the combustion chamber of the turbojet engine, and generally ends with a jet nozzle whereof the output is situated downstream of the turbojet engine.
The role of these thrust reverser means is, during landing of an airplane, to improve the braking capacity thereof by reorienting at least part of the thrust generated by the turbojet engine forward. During this phase, the reverser obstructs at least part of the stream of the cold flow and orients that flow toward the front of the nacelle, thereby generating a counter-thrust that is added to the braking of the wheels of the airplane.
One common thrust reverser structure comprises a cowling in which an opening is formed designed for the deviated flow which, in a direct thrust situation of the gases, is closed by a sliding cowl and which, in the thrust reversal situation, is freed by translating the sliding cowl in the downstream direction (in reference to the flow direction of the gases), using cylinders for moving the cowl mounted on a frame of the cowling upstream from the opening, called front frame.
Because the nacelle undergoes axial aerodynamic forces during flight tending to cause the structure to retract relative to the engine, this front frame is connected to the structure of the turbojet engine, and, more specifically, the fan casing by means of connecting flanges or connections of the blade/groove type, for example.
The sliding cowl can be formed by an outer assembly made in a single piece with no breaks in the lower portion thereof slidingly mounted on rails positioned on either side of the pylori of the aircraft between a direct jet position and a thrust reversal position.
Such a cowl is often designated using the term “O-duct,” which refers to the shape of the shroud of such a cowl, as opposed to a “D-duct,” which comprises two half-cowls each extending over a half-circumference of the nacelle.
It is of course crucial for a sliding movement of the cowl using a thrust reverser not to occur unexpectedly: such opening would in fact be critical during a flight phase.
For these reasons, safety bolts are provided at various locations of the thrust reverser to prevent unwanted opening of the cowl.
In a “D-duct” reverser, three safety bolts are traditionally provided for each half-cowl or for both half-cowls if they are mechanically connected to each other. Two primary bolts are typically positioned on the front frame to act directly on two actuating cylinders of each half-cowl.
However, these primary bolts may become inoperative following the rupture of a rotor or a blade of the turbojet engine, that rupture causing the projection of debris around the turbojet engine, said debris impacting the reverser and being able to deform the reverser and/or deteriorate the bolts.
To quite significantly reduce the risk of unexpected opening, a third bolt is then available that ensures that the reverser is kept closed following the loss of the other two lines of defense after the aforementioned ruptures, this third bolt being inserted between the so-called “six o'clock” lower beam (i.e., positioned in the lower portion of the nacelle and on which the two half-cowls are slidingly mounted) and the concerned half-cowl.
The remote location of the third bolt with respect to the other two primary bolts offers increased safety with respect to a “rotor burst” (explosion of a disc of the rotor of the turbojet engine) or a blade-out.
In such a case, only one or two bolts may potentially be destroyed by the same disc, but not all of them.
A force path is thus preserved between the reactor mast and the lower beam. If that lower beam is cut, a force path will remain connecting the reactor mast to the bolt owing to the presence of the inner structure, called IFS, which connects the upper and lower beams over the entire length thereof.
In the case of an O-duct reverser, a similar arrangement would be desirable despite the absence of the lower beam.
It is thus possible to consider positioning a third line of defense and/or an inhibiting device between the front frame and the sliding cowl.
However, these bolts are bulky and become difficult to position when the reverser is very thin, i.e., the distance between the inner cowling and the outer cowling is reduced.
Irrespective of the arrangement provided for these bolts, the risk of the reverser, and more particularly the interface of the front frame and the fan casing, deforming and deteriorating following a rotor disc explosion is not nonexistent, making the installation of a third line of defense between the front frame and the sliding cowl ineffective.
In fact, the burst of an engine disc results in the discharge of a disc third with energy considered to be infinite, intermediate fragments (smaller disc portions) with significant energy, and small fragments (generally turbine or compressor blade elements) with low energy.
Because the intermediate fragments can be discharged over the entire circumference of the cowl, there is a risk of the retaining device between the front frame and the fan casing not being sufficient to retain the thrust reverser cowl axially.
To resolve this problem, it is known to place the third line of defense, i.e., the third bolt, at the rear end of the rails supporting the cowl, inserted between said pylori and said cowl. For the same reasons, the mechanical inhibiting device of the thrust reverser is placed in the same area.
The presence of these locking means between the cowl and the pylori makes it possible to perform geographically independent locking of the interface between the front frame and the fan casing, thereby offering the desired degrees of reliability and safety.
However, this type of assembly causes access and visual verification difficulties, which is in particular problematic when using a mechanical inhibiting device of the thrust reverser positioned in that location.